Verify the breakpoint is not disabled. Using the Run | View Breakpoints... menu (Ctrl+Shift+F8 / CMD+Shift+F8), check that breakpoints are enabled. Tick the checkbox in front of a breakpoint to enable it.

On Mac OS X when renaming a file or folder using Finder and changing the letter case, PhpStorm may not be able to find the files. If a rename is required, make sure to do it in the IDE or using a terminal and the mv command. Renaming project files or folders using Finder may result in strange behaviour. See http://devnet.jetbrains.com/message/5488439#5488439 for a full discussion about this.

There are some rare cases when breakpoints can't be hit due to technical limitations, e.g. caused by the way how PHP generates byte-code (see this comment for more information).